Nitrogen accumulation in a forest ecosystem: the role of legume under storey
2005
Wanjiku, J.(Kenya Agricultural Research Institute, Nairobi (Kenya). National Agricultural Research Laboratories)
Though inorganic fertiliser may be used to ameliorate N deficiencies, this option is inefficient as trees recover less than 30% of applied N (Thomas and Mead, 1992), indicating substantial leakage of N to the ground water. In the tropics, the use of inorganic fertilizers in forest is furt by the low economic standards. Hence, there is need to promote low cost sustainable approaches to soil fertility improvement in forest ecosystems that may include biological nitrogen fixation. Nitrogen fixing legumes have great potential for maintaining and increasing soil fertility in forest and cropping ecosystems. Though attractive, the use of biological nitrogen fixation (BNF) as an alternative to inorganic fertilizers poses problems to land managers, as the technology is complex. For instance, the cycling and the fate of fixed N, its transfer in many instances are not well understood. In an attempt to understand the fate of fixed N in a sandy soil ecosystem, a study was conducted after four years of a legume growth under a young Pinus radiata plantation. Nitrogen accumulation in the above- and below-ground biomass as well as in the soil profile was quantified in order to trace the movement of the N - fixed. Of the total N in the above-ground legume biomass, 95% was derived from fixation, this amounted to 214 Kg N - fixed/ha/yr. Of this nitrogen, only 16% (34.5kg N/ha/yr) and 18.7% (40kg N/ha/yr) was accumulated in the above- and below-ground biomass, respectively. A large proportion, 65.2% (139.5 Kg N !ha/yr) of the N-fixed could not be accounted for in the materials tested. Much of the fixed N was probably lost through leaching in the free draining soil. In such an ecosystem it is important to retain the N-fixed by introducing a companion crop that is capable of storing the fixed N.
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